MCAT Biochemistry 2 — Questions and Answers
Question 1: Which enzyme catalyzes the rate-limiting step of the citric acid cycle?
- Citrate synthase
- Isocitrate dehydrogenase (Correct answer)
- Alpha-ketoglutarate dehydrogenase
- Succinate dehydrogenase
Correct answer: Isocitrate dehydrogenase
Isocitrate dehydrogenase is the primary regulatory enzyme of the TCA cycle, inhibited by ATP and NADH and activated by ADP and Ca²⁺.
Question 2: A patient presents with elevated homocysteine levels. Which vitamin deficiency is most likely responsible?
- Vitamin B1 (thiamine)
- Vitamin B6 (pyridoxine)
- Vitamin B12 (cobalamin) (Correct answer)
- Vitamin C (ascorbate)
Correct answer: Vitamin B12 (cobalamin)
Vitamin B12 is required for the conversion of homocysteine to methionine; its deficiency causes homocysteine accumulation.
Question 3: Which glycolytic intermediate is also a precursor for the synthesis of serine?
- Fructose-6-phosphate
- Glucose-6-phosphate
- 3-Phosphoglycerate (Correct answer)
- Phosphoenolpyruvate
Correct answer: 3-Phosphoglycerate
3-Phosphoglycerate is diverted from glycolysis to form serine via the phosphoserine pathway.
Question 4: In fatty acid beta-oxidation, which coenzyme directly accepts electrons in the first oxidation step?
- NAD⁺
- FAD (Correct answer)
- NADP⁺
- CoA
Correct answer: FAD
The first oxidation step of beta-oxidation is catalyzed by acyl-CoA dehydrogenase, which uses FAD as the electron acceptor.
Question 5: Which of the following correctly describes a competitive inhibitor's effect on enzyme kinetics?
- Increases Km, decreases Vmax
- Decreases Km, increases Vmax
- Increases Km, no change in Vmax (Correct answer)
- No change in Km, decreases Vmax
Correct answer: Increases Km, no change in Vmax
Competitive inhibitors increase the apparent Km (reducing substrate affinity) but do not alter Vmax because excess substrate can outcompete the inhibitor.
Question 6: Maple syrup urine disease results from deficiency of which enzyme complex?
- Pyruvate dehydrogenase
- Alpha-ketoglutarate dehydrogenase
- Branched-chain alpha-keto acid dehydrogenase (Correct answer)
- Propionyl-CoA carboxylase
Correct answer: Branched-chain alpha-keto acid dehydrogenase
Branched-chain alpha-keto acid dehydrogenase deficiency leads to accumulation of leucine, isoleucine, and valine metabolites, causing characteristic sweet-smelling urine.
Question 7: Which linkage type is found in cellulose but NOT in starch?
- α-1,4-glycosidic
- β-1,4-glycosidic (Correct answer)
- α-1,6-glycosidic
- β-1,6-glycosidic
Correct answer: β-1,4-glycosidic
Cellulose consists of glucose monomers joined by β-1,4-glycosidic bonds, which humans cannot digest, unlike the α-1,4 bonds in starch.
MCAT Biochemistry 2
Which enzyme catalyzes the rate-limiting step of the citric acid cycle?